Vapor pressure and specific electrical conductivity in the solid and molten H₂O-CsH₂PO₄-CsPO₃ system—a novel electrolyte for water electrolysis at ~ 225–400 °C

Cesium dihydrogen phosphate, CsH₂PO₄ (CDP) was studied for water electrolysis at ~ 225–400 °C. In the presence of sufficient humidity, CDP is structurally disordered and super-protonic conducting with conductivities reaching 0.2–0.25 S cm⁻¹, when determined in suitable H-shaped sealed conductivity cells. Freshly prepared 99.7 ± 0.3% gravimetric pure CDP with correct X-ray diffraction and DSC diagram melted at ~ 345 °C. The vapor pressures, above CDP alone and mixed with 20–50 mol% CsPO₃ or 13 mol% H₂O, were determined in sealed ampoules up to 355 °C by means of Raman spectroscopy based on internal reference gases. Pressures up to ~ 49 bar were estimated, much higher than previously expected. Conductivities were given as polynomials and plotted in solid and liquid states. Water splitting electrolysis 2H₂O → 2H₂ + O₂ was demonstrated by Raman at ~ 355 °C under a water pressure of ~ 23 bar in a quartz cell with platinum electrodes, showing molten CDP to have significant potential for water electrolysis.